Secondary mineral–fluid equilibria in the Krafla and Námafjall geothermal systems, Iceland

Aquifer fluid compositions and aqueous species distribution have been calculated for 22 samples collected from producing wells in the Krafla and Námafjall geothermal areas, Iceland. Overall mineral–solution equilibrium is rather closely approached in the aquifer beyond the depressurization zone around wells for all major components entering hydrothermal minerals. The minerals involved are in alphabetical order: albite, calcite, chlorite, epidote, K-feldspar (adularia), prehnite, pyrite, pyrrhotite, quartz and wairakite. Calculated saturation indices for OH-bearing Al–silicates show more scatter than for minerals of simpler composition. A large scatter is also observed for pyrite and pyrrhotite. The main uncertainty involved in calculating mineral saturation indices, particularly in the case of “excess” enthalpy well discharges, lies in the model adopted to calculate the aquifer water composition and its aqueous species distribution and not in the quality of the thermodynamic data on the aqueous species and the minerals with the possible exception of epidote and Fe2+.